1. Field of the Invention
This invention relates to welded joints of welded tubular structures for use in piping of chemical plants or nuclear power stations, and more particularly to the structure of a welded joint of stainless steel tubular members and a method for producing such structure which best suits the purpose of avoiding the leak of a corrosive medium to outside through cracks formed in the welded joint of tubular members formed of austenite stainless steel tending to develop stress corrosion cracking.
2. Description of the Prior Art
Generally, austenite stainless steel shows precipitation of carbides in the grain boundary when heated to 500.degree. to 800.degree. C. The precipitation of carbides is recognized in members joined by welding although these members are exposed to elevated temperatures for a short period of time. When residual stresses are produced in the heated zones or tensile stresses are applied from outside to the heated zones, such zones will tend to develop stress corrosion cracking in specific corrosion inducing circumstances. The present practice in the art of production of welded tubular structures of austenite stainless steel is to join tubular members of such material by butt welding, and such structures show the tendency to develop stress corrosion cracking. In nuclear power stations where an extraordinary degree of caution is required in the operation of the reactors to prevent accidents, such unstable tubular structures in the piping for connecting various apparatus cannot be left as they stand.
There has in recent years arisen a problem involving the welded joint of tubular members of austenite stainless steel which has attracted a considerable degree of attention, and various measures to solve the problem have been proposed. For example, the piping for connecting apparatus in a nuclear power station is exposed to pure water which is a coolant flowing through the nuclear reactor, and there are possibilities that the aforesaid stress corrosion cracking will be caused by the pure water. More specifically, in a welded tubular structure of austenite stainless steel, tensile residual stresses are produced and welding heat affected zones (hereinafter to be referred to as HAZs) of high stress corrosion cracking sensitivity are formed in the vicinity of the welded joint, thereby presenting a condition in which stress corrosion cracking tends to develop.
As a method for preventing the development of stress corrosion cracking in the welded joint of tubular members of austenite stainless steel under corrosion inducing circumstances, proposals have been made to use a welding process for performing welding while forcedly cooling the interior of the steel tubular members in order to reduce the residual stresses formed in the welded joint and the precipitation of carbides therein. The residual stresses in the weld inside the tubular members are compressive stresses when this welding process is used, so that this process is very effective in reducing the chances of occurrence of stress corrosion cracking in cases where the inner surfaces of the tubular members are exposed to corrosion inducing atmosphere.
However, it is evident that the welded joint formed by welding tubular members while allowing the tubular members to cool by natural cooling tends to develop stress corrosion cracking as aforesaid if no measures are taken for preventing this phenomenon. No effective measures to cope with this situation have yet been established.
In U.S. Pat. No. 4,049,186 to Hanneman et al., there is shown a proposal to perform overlay welding on the butt welded joint and its vicinity of tubular members of austenite stainless steel on part of or the entire outer surfaces of the tubular members. In this welding process, it requires a lot of labor to perform overlay welding on the entire outer surfaces of the tubular members. However, when overlay welding is performed on part of the outer surfaces, the problem arises of stress corrosion cracking occurring in portions where no overlay welding is performed. Moreover, in this welding process, it is essential to avoid the use of materials which will have unfavorable influences on the tubular members in performing welding because overlay welding is performed directly on the tubular members. Thus, limitations are placed on the welding materials that can be used because a material used must be such that it enables overlay welding suitable for steel tubular members to be effected.